The invention relates to materials of construction suitable for electrochemical double-layer capacitor components of improved structure and to processes for preparing them and component parts utilizing them.
Double-layer electrochemical capacitors provide energy storage and pulse power delivery for a number of stationary and mobile power needs. Supercapacitors can store and release large bursts of energy and can be useful in many environments including external power sources and as complements to power sources such as fuel cells and the like. The unique advantages of these devices make them promising for many power applications. To maximize their usefulness, a balance must be struck between weight and performance, and it would be desirable to adjust manufacturing procedures of current construction materials to assure that both concerns are effectively addressed to provide a net improvement in the operation and/or economy of these devices.
Double-layer capacitors, sometimes also called electrochemical ultracapacitors or supercapacitors, are capable of rapidly charging to store significant amounts of energy and then delivering the stored energy in bursts on demand. To be useful, they must, among other properties, have low internal resistance, store large amounts of charge and be physically strong per unit weight. There are, therefore, a large number of design parameters that must be considered in their construction. It would be desirable to enable procedures for producing starting materials for producing component parts that would address these concerns such that the final supercapacitor assembly could be more effective on a weight and/or cost basis.
Double-layer capacitors generally include two porous electrodes, kept from electrical contact by a porous separator. Both the separator and the electrodes are immersed within an electrolyte solution. The electrolyte is free to flow through the separator, which is designed to prevent electrical contact between the electrodes and creating a short circuit in the cell. Current collecting plates are in contact with the backs of active electrodes. Electrostatic energy is stored in polarized liquid layers, which form when a potential is applied across two of the electrodes. A double layer of positive and negative charges is formed at the electrode-electrolyte interface.
The use of graphite electrodes in electrochemical capacitors with high power and energy density provides a number of advantages, but economics and operating efficiency are in need of improvement. Fabrication of double layer capacitors with carbon electrodes is known. See, for example, U.S. Pat. No. 6,094,788, to Farahmandhi, et al., U.S. Pat. No. 5,859,761, to Aoki, et al., U.S. Pat. No. 2,800,616, to Becker, and U.S. Pat. No. 3,648,126, to Boos, et al. The art has been utilizing graphite electrodesxe2x80x94but not flexible graphite sheetsxe2x80x94for capacitors of this type for some time and is still facing challenges in terms of material selection and processing. While various attempts have been made to achieve good electrical, physical and electrochemical properties, the art is in need of improvements and the provision of alternative electrochemical capacitors and components effective for various arrangements of electrochemical elements and materials.
A continuing problem in many carbon electrode capacitors, including double-layer capacitors, is that the performance of the capacitor is limited because of the internal resistance of the carbon electrodes. While the use of carbon in the form of flexible graphite sheet has several advantages, it is desired to further reduce cell internal resistance. Internal resistance is influenced by several factors, including the high contact resistance of the internal carbon-carbon contacts, the contact resistance of the electrodes with a current collector, the surface and internal pore structure of the carbon and the material thickness. Because high resistance translates to large energy losses in the capacitor during charging and discharge, and these losses further adversely affect the characteristic RC (resistance x capacitance) time constant of the capacitor and interfere with its ability to be efficiently charged and/or discharged in a short period of time, it would be desirable to provide construction materials and methods that would facilitate reductions in the internal resistance.
There remains a need in the art for a material which can be used in preparing electrochemical double-layer capacitors having improved properties due to the use of novel components and processing.
Accordingly, it is an object of the invention to provide a material that can be used in preparing a flexible graphite articles useful as components in electrochemical supercapcitors.
It is an object of the invention to resolve some of the art recognized problems in the construction and operation of electrochemical double-layer capacitors as outlined, in part, above and elsewhere.
It is a general object of the invention to provide improved electrochemical capacitors.
It is another general object of the invention to provide improved components for electrochemical double-layer capacitors.
It is another object of some embodiments of the invention to provide improved components for electrochemical double-layer capacitors that are stable in the presence of strong acids.
These and other objects are accomplished by the present invention, which provides electrochemical double-layer capacitors.
In one aspect, the invention provides an electrochemical double-layer capacitor comprising: two paired electrodes wetted with an electrolyte, each electrode being in contact with a current collector comprised of flexible graphite sheet and being separated from the other of the paired electrodes by a separator layer porous to the electrolyte. The current collector further can comprise a layer of tanged metal, and in some embodiments is integral with the electrode. In some embodiments, the electrode comprises a fibrous carbon material such as carbon fabric or carbon paper. In other embodiments, the electrode comprises flexible graphite sheet, which can be impregnated with a material, e.g., a metal, effective to modify its conductivity.
In another aspect, the invention provides an electrochemical double-layer capacitor comprising: two paired electrodes comprising graphite, both wetted with an electrolyte, each electrode being in contact with a current collector comprising tanged metal and being separated from the other of the paired electrodes by a separator layer porous to the electrolyte. In some embodiments, the graphite comprises a fibrous material such as carbon fabric or carbon paper. In other embodiments, the electrode comprises flexible graphite sheet. In some embodiments, the graphite is impregnated with a material, e.g., a metal, effective to modify its conductivity.
In another aspect the invention provides an electrochemical double-layer capacitor comprising: two paired electrodes comprising flexible graphite, both wetted with an electrolyte, each electrode being in contact with a current collector and being separated from the other of the paired electrodes by a separator layer porous to the electrolyte. In some embodiments, the electrode is integral with the current collector. In some embodiments, the current collector comprises a layer of tanged metal. In certain embodiments, the electrode is integral with the current collector and can be impregnated with a material effective to modify its conductivity.
In another aspect, the invention provides improvements in an electrochemical double-layer capacitor of the type comprising at least two electrodes wetted with an electrolyte, each electrode being in contact with a current collector and being separated from an adjacent electrode by a separator layer porous to the electrolyte, the improvement comprising: as an electrode, a current collector or a combined electrode and current collector, an integral sheet of tanged metal and flexible graphite. In some embodiments, the composite is comprised in an electrode. In some, the composite is comprised in a combined current collector and electrode. And, in others, the composite is comprised in a current collector.
According to the process of the invention, composite materials useful as components in electrochemical supercapcitors and as described above are processed in the manner described below.
Many preferred and alternative aspects of the invention, including preferred components and aspects of processing are described below.